Numerical comparison of an office cooled with and without a ventilated slab using a model predictive controller

Abstract

Ventilated slabs are Thermally Activated Building Systems (TABS) that use the supply air as the heating/cooling medium for taking advantage of the thermal inertia of a slab. The objective for the ventilated slab is to maintain the indoor temperature within the desired range for a minimal energy consumption, at least equal or lower than with a regular (non-ventilated) slab. To achieve such objectives, an efficient control is required but the latter is challenging for TABS because of the delay brought by their thermal inertia. While advanced control techniques are available and suitable with TABS, it is not always clear if the final performances are driven by the control technique itself or the inherent thermal potential of the ventilated slab. The present paper numerically analyses the behaviour of the ventilated slabs controlled with a Model Predictive Controller, and provides a comparison with a non-ventilated slab combined with the same controller. This comparison was extended to four other airflow rates, since it significantly changes the thermal dynamic of a ventilated slab. The indoor temperature requirements were satisfied most of the time and energy demand differences observed between the two systems were minor. However, a lower airflow rate is preferable as it decreases the consumption of the fans, which was significant for this test case. The most important improvement brought by the ventilated slab was the smoothed temperature variation at the outlet of the slab, resulting in a smaller blown air – room air temperature difference with ventilated slabs and thus favouring indoor comfort.